Processing a workpiece may be performed by controlling the position and movement of a tool relative to the workpiece using a numerical control (NC) device. During processing of the workpiece, the tool is moved relative to the workpiece along a tool path that is typically generated according to computer-aided design (CAD) or computer-aided manufacturing (CAM) data related to a desired geometry of the workpiece. The tool path is encoded by path-describing NC data that are stored in the NC device and processed by an NC program to process the workpiece.
Often times, a geometry of the processed workpiece deviates from the CAD geometry (i.e., the desired geometry). For example, workpieces that have undergone a forming process (e.g., deep-drawn workpiece parts) may exhibit relatively large inaccuracies that make it difficult to process the workpieces along a fixed, desired tool path specified by the NC device.
In an example of processing a workpiece using a laser processing apparatus, a laser processing head of the laser processing apparatus must maintain a constant working distance (e.g., 1 mm) from the workpiece. However, since the workpiece inaccuracies are often larger than the working distance, the particular working distance between the laser processing head and the workpiece to be processed is measured using a distance measuring device. The distance measuring device is typically integrated with the laser processing head. The working distance is then regulated by the NC device such that the working distance is set to a specified desired distance based on the measured distance values. That control loop is superimposed on the motion control of the laser processing head along the desired tool path specified by the NC device. Such superposition is referred to as distance control.
The value of a regulation parameter generally depends on a deviation of the manipulated variable from a desired value. Values of the regulation parameters are generally relatively low, with the result that relatively large inaccuracies of the workpiece can have a negative effect on the processing. If the workpiece inaccuracies exceed a certain magnitude, the distance control is unable to correct the difference sufficiently, which may result in collisions between the tool and the workpiece and termination of the process. In such cases, the desired tool path specified by the NC program may need to be corrected manually. Correcting the tool path may take place during essential operating time of the workpiece processing apparatus and therefore be costly to the process.